Theoretical Study Of The Photoassociation And Photodissociation Dynamics Of Diatomic Molecules

With the rapid development of the laser technology,the interactions of laser fields with molecules have been receiving more and more attention from researchers,because of the widespread applications in various areas including detecting material structure,quantum calculation,controlling chemical reaction,atmospheric environmental improvement,nation-al defense military construction,etc.There is no rigorous definition for a laser field to be"weak" or "strong".Generally,according to the extent of the interaction of the laser field with the molecule,one classifies laser fields to be the weak field(<1010 W/cm2),the moderate-intensity field(1011?1013 W/cm2),the strong field(1013?1015 W/(cm2),the super-strong field(>1015 W/cm2).Most of the effort has been devoted to the study of the interaction of the strong field with molecules,because in the strong field,the nuclear dynamics present strong correlations with the electrons,which may result in many novel non-linear physical phenomena,such as the multiphoton transition,above threshold ionization,above threshold dissociation,high-order harmonic generation,etc.However,it is seldom reported whether in the moderate-intensity laser field,the molecules could also present the similar non-linear phe-nomena.Focusing on this topic,based on the time-dependent quantum wavepacket method,we investigated the photoassociation and photodissociation dynamics of several elemental di-atomicmolecules in moderate-intensity laser fields.It has been found that by properly setting the initial state,for example,the initial collision momentum or the initial vibrational state,one could still observe significant non-linear phenomena including the multiphoton transition,above threshold dissociation,etc.This is of importance in various aspects,including enrich-ing the theory of field-molecule interaction and achieving laser control of molecular quantum states.The major achievements of this thesis have been listed as follows.(1)We explored the vibrational state-selective photoassociation in the ground state of the HX(X=F,Cl,I)moleculesFor the three systems,the photoassociation probability was calculated and compared by considering different initial collision momentums.It has been found that out of the three systems,the photoassociation probability of the HF system is the largest and the multiphoton transition is the most probably to occur.In addition,the photoassociation probabilities are in accordance with the overlap of the initial continuum with the final target vibrational state for the HI and HCl systems,but it is not the case for the HF system due to its strong multiphoton transition probability.(2)The time-independent Schrodinger equation for the HD+ molecule was solved beyond the Born-Oppenheimer approximation.In the adiabatic representation,the wavefunction of the ground vibrational eigenstate was found to contain two parts:One is on the ground 2?g+(1s?g)state which is dominant,and the other is on the repulsive excited 2?u+(2p?u)state in the range from R=.0?5.0 a.u..This is because the nonadiabatic coupling between the ground 2?g+(1s?g)and excited 2?u+(2p?u)states is strong in that region.The component of the excited 2?u+(2p?u)state of the vibrational eigenstate can be approximately expanded with vibrational wavefunctions of the ground 2?g+(1s?g)state.(3)The above threshold dissociation of the molecular ion HD+ in moderate-intensity laser pulse fields has been investigated.The time of flight(TOF)neutral atom spectra pro-duced from dissociation of the v=10 vibrational state at the laser intensity of 1012 W/cm2 and from v=0 at the intensity of 5 × 1012 W/cm2 were calculated,respectively.The calcula-tions results agreed well with previous experimental results.Furthermore,the kinetic energy distribution of the dissociated fragments of molecular ion HD+ have been calculated within the moderate laser intensity region by considering initial vibrational states from v=0 to 15.It has been found that the above threshold dissociation could be more obviously observed from the TOF spectra when the initial vibrational state is set as v=6 in a moderate-intensity femtosecond laser field.(4)By t,aking the HD+ system as an example,the first-order dynamic Stark effect.(DSE)and the intrinsic nonadiabatic coupling(INAC)effect on the photodissociation dynamics have been investigated theoretically.The energy difference between two dressed-state potential en-ergy curves around R= 10 a.u.can be equal to zero due to the first-order Stark shift,which results in a new passage connecting the excited state with the ground state.Moreover,the population transfer passage resulting from the INAC effect can be suppressed by the DSE which enlarges the energy gap betwee two dressed states at appropriate laser intensity and pulse duration.